Conventionally, acoustic apparatuses having improved tone quality have been developed. In the acoustic apparatuses, particularly, the improvement of acoustic properties, such as an output acoustic pressure level, distortion ratio, and flatness, is desired. In recent years, it is believed that the acoustic properties depend on various physical properties of a housing of the acoustic apparatus, and hence the physical properties of the acoustic apparatus housing are considered to be important. When an acoustic apparatus is driven to generate acoustic waves, the acoustic apparatus housing generally vibrates in resonance with the acoustic waves. Consequently, the resonance causes the tone quality of the acoustic apparatus to be poor. For solving this problem, it is considered that a rigid material having an appropriately large internal loss is used in the acoustic apparatus to improve the acoustic properties including an output acoustic pressure level, distortion ratio, and flatness. Therefore, a wood board, such as lauan made of wood as a raw material, or a particle board formed from wood pieces bonded by an adhesive, has been used as a material for the acoustic apparatus housing.
However, when the wood board is used as a material for acoustic apparatus housing, the wood board has a small degree of freedom in the outer shape and hence restricts its use, and thus the wood board does not meet the demands. Further, the wood board needs a step for assembling wood boards processed into desired forms, and hence lowers the productivity, and thus the wood board does not satisfy the requirement in respect of productivity. For solving these problems, as a material for acoustic apparatus housing, a material obtained by mixing a thermoplastic or thermosetting resin with inorganic filler and subjecting the resultant mixture to injection molding is studied and used (Japanese Patent Application Publication No. JP H06-169498). The resultant acoustic apparatus housing is improved in rigidity due to the inorganic filler; however, the acoustic apparatus housing has low internal loss and hence exhibits poor tone quality than that of a wooden housing. On the other hand, when an ABS resin is used instead of polypropylene (PP) in the material for acoustic apparatus housing, the resultant acoustic apparatus housing has a small internal loss and thus undergoes resonance, causing distortion, and therefore this material is not suitable for acoustic apparatus housing.
On the other hand, from the viewpoint of protecting the environment, conventionally, a resin derived from petroleum as noble fossil resources is frequently used as a material for acoustic apparatus housing, and it is expected that a substitute for the petroleum resin is rapidly developed for preventing the exhaustion of resources. Recently, a variety of synthetic resin materials are developed as materials for acoustic apparatus housing and provided, and the amount of the synthetic resin materials used in the field of acoustic apparatus housing increases year by year. Consequently, the amount of synthetic resin waste also increases, and the method for treatment of the resin waste leads to serious social problems. When the resins disposed of are directly subjected to thermal treatment, it is possible that harmful gas, such as dioxin, is generated or the incinerator suffers a damage due to large heat of combustion, leading to a danger of imposing a heavy burden on the environment.
As examples of conventionally known methods for treatment of waste resin, which are unlikely to impose a burden on the environment, there can be mentioned methods in which molecular weight of the waste resin is lowered by thermal decomposition or chemical decomposition and then incinerated or buried. However, the thermal disposal generates exhaust carbon dioxide and therefore possibly causes global warming, and, when a resin containing sulfur, nitrogen, or halogen is incinerated, exhaust harmful gas may be generated to cause air pollution. Currently, most of the resins used in the acoustic apparatus housing remain intact for a long term without decomposing after being disposed of and buried, and they may cause soil pollution or water pollution. For solving the problem, biodegradable resins have been developed recently and attracted attention, and put into practical use as disposal products, for example, mainly, utensils for daily use, sanitary goods, and play tools.
As mentioned above, it is known that the acoustic apparatus housing using wood has excellent tone quality. Therefore, as a non-natural material for acoustic apparatus housing, which is improved to have tone quality equivalent to or higher than that of the housing using wood, for example, a material obtained by mixing cellulose filler, such as wood meal or rice hull, with a resin is studied and used (Unexamined Japanese Patent Application Laid-Open Specification No. Hei 10-164679). In this case, the cellulose filler is surface-treated with phenol or the like for improving the compatibility with the resin. The cellulose filler, however, does not have satisfactory lubricating properties, as compared to general inorganic filler, and therefore only a limited amount of the cellulose filler can be added to the resin, making it difficult to improve the acoustic apparatus housing in rigidity.
In addition, when a loudspeaker is driven at a predetermined power, the acoustic apparatus housing using the cellulose filler vibrates itself in resonance with the loudspeaker vibration, so that the acoustic pressure frequency properties of the loudspeaker are not stabilized, causing deviation. This disadvantageously increases distortion in the acoustic apparatus housing. Further, the cellulose filler forms lignin or pyrolignous acid when being mixed with the resin and shaped. Therefore, the acoustic apparatus housing using the cellulose filler may suffer corrosion, and hence this material does not satisfy the requirements of the material for acoustic apparatus housing. Thus, there has been desired a material having excellent performance as an acoustic apparatus housing, which is unlikely to impose a burden on the environment and solves the above problems of corrosion and the like.
A task of the present invention is to provide a material which is advantageous not only in that it is unlikely to impose a burden on the environment and prevents the resin from suffering corrosion, but also in that it has excellent performance as an acoustic apparatus housing.